This invention relates to an improvement in switching amplifiers and particularly to an improvement in the driver circuit for the power transistors thereof.
Typically, switching amplifiers have power (or output) transistors that are controlled fully "on" (saturated) or fully "off" (reverse biased). The power transistors are controlled directly or through driver circuits by a pulse width modulator (PWM) of some suitable variety. The duty cycle of the PWM output pulse chain bears a direct relationship to an analog input (or control) signal. In this way, the average "on" time of the power transistors is directly related to the analog signal input to the PWM.
As is well known in the art, switching amplifiers may have only one power transistor. (See British Pat. No. 862,981 (1961).) They may have two power transistors, for example, back-to-back between power supplies of opposite polarity. (See Crane U.S. Pat. No. 3,054,066, Schmid U.S. Pat. No. 2,951,212 and British Specification No. 853,340 (1960).) They may have four power transistors arranged in a bridge configuration. (See Gregory U.S. Pat. No. 3,260,912 and "Switching Inductive Currents With Power Transistors" by R. Colclaser Electro-Technology, October 1962.) This invention pertains to the driver circuit or circuits between the PWM and the power transistor or transistors for any switching amplifier.
As is well understood in the art, switching amplifiers may be self-switching or may be oscillator controlled. Self-switching amplifiers (see Bose U.S. Pat. No. 3,294,981) are typically variable output frequency devices whereas oscillator controlled switching amplifiers are normally fixed frequency output devices. Typically, the frequency is selected in excess of the audible range.
An advantage of this invention is to provide a switching amplifier with high switching rates, say 5 to 30 kilohertz in the case of bipolar output transistors and up to about 100 kilohertz in the case of MOSFET output transistors.
It is another advantage of this invention to provide a switching amplifier characterized by low power consumption due to predominently FET and MOSFET design.
It is a special advantage of this invention to provide a switching amplifier the operation of which is independent of the bus voltage applied to the power transistors and flyback diodes.
It is still another advantage according to this invention to provide a switching amplifier with a high degree of electrical isolation between the PWM control and the final driver circuit.
It is yet another advantage according to this invention to provide a switching amplifier having low cost and low component count.
It is yet another advantage according to this invention to provide a switching amplifier having a driver circuit with a very fast rising and falling drive signal, the rate of the rise of the driver signal being easily adjusted by component selection.